1. Field of the Invention
The subject invention generally relates to an article having an impact resistant surface for preventing damage to the article upon impact by an object.
2. Description of the Related Art
With the increased cost of metals, such as aluminum and steel, various articles previously manufactured from metals such as steel, cast aluminum, and steel/plastic composites are now being manufactured from a polymeric material. The articles are not only less expensive to produce from the polymeric material, but also reduce mass. However, the article must meet predetermined design requirements. The design requirements include impact resistance, i.e., the articles must still be capable of withstanding an impact from an object without fracturing. As such, the polymeric material is typically filled with glass fibers to increase the strength of the article. In order to meet the required impact resistance, longitudinally extending ribs are typically incorporated into an exterior surface of the article. The ribs are exposed to the impact from the object. These longitudinally extending ribs are integrally formed with the article, and extend in parallel rows along a length of the article. The ribs increase the geometric strength (rigidity) of the article, i.e., the ribs increase resistance to bending or flexing.
The glass fibers typically align themselves with a direction of flow of the polymeric material as the article is being formed, i.e., the glass fibers align with the direction of flow of the polymeric material being injected into a mold. A strength of the article comprising the glass fibers is greatest when a load is applied in the direction of the orientation of the glass fibers, i.e., parallel to the orientation of the glass fibers, and is least when the loading is applied in a direction perpendicular to the orientation of the glass fibers. A resistance to elongation of the article is least when the loading is applied in the direction of the orientation of the glass fibers, i.e., parallel to the orientation of the glass fibers, and is greatest when the loading is applied in a direction perpendicular to the orientation of the glass fibers. Therefore, the resistance to elongation acts opposite the strength, with the resistance to elongation of the article being highest when the loading is applied perpendicular to the orientation of the glass fibers in the article and the strength of the article being highest when the loading is applied parallel to the orientation of the glass fibers in the article.
The overall impact resistance of the article is dependent upon both the strength and the resistance to elongation of the article. Therefore, a longitudinal rib pattern in which the ribs are aligned parallel with the orientation of the glass fibers in the article maximizes the bending strength, but minimizes the resistance to elongation, whereas a longitudinal rib pattern in which the ribs are aligned perpendicular with the orientation of the glass fibers in the article minimizes the bending strength and maximizes the resistance to elongation.
An example of an article previously manufactured from steel that is now manufactured from the polymeric material is an oil pan (fluid reservoir) for an internal combustion engine. The longitudinal ribs run substantially along the entire length of the oil pan, such that the longitudinal ribs extend along a longitudinal axis of a vehicle and parallel with a direction of travel of the vehicle. As such, the object, for example a stone or some other debris, will most likely be traveling in a direction parallel the longitudinal ribs.
As known in the prior art, each of the longitudinal ribs include a pair of side surfaces in spaced parallel relationship defining a generally rectangular cross section. Each of the longitudinal ribs extends upward from an exterior surface of the oil pan, with the sidewalls intersecting the exterior surface at an inner corner, i.e., a vertex having an approximate angle of 90°. In other words, the ribs are substantially perpendicular to the exterior surface of the oil pan. Upon impact by the object, the substantially perpendicular intersection between the side surfaces of the ribs and the exterior surface of the oil pan creates a concentrated stress point in the exterior surface of the oil pan at the vertex of the inner corner. While the longitudinal ribs increase the impact resistance of the oil pan, the oil pan remains susceptible to fracture at these concentrated stress points located at the intersections of the side surfaces of the longitudinal ribs and the exterior surface of the oil pan. Accordingly, there remains a need to further increase the impact resistance of these various articles.